Curable thermoset coating compositions are widely used in the coatings art. They are often used as topcoats in the automotive and industrial coatings industry. Such topcoats may be basecoats, clearcoats, or mixtures thereof. Color-plus-clear composite coatings are particularly useful as topcoats where exceptional gloss, depth of color, distinctness of image, or special metallic effect is desired. The automotive industry has made extensive use of these coatings for automotive body panels.
Color-plus-clear composite coatings, however, require an extremely high degree of clarity in the clearcoat to achieve the desired visual effect. High-gloss coatings also require a low degree of visual aberrations at the surface of the coating in order to achieve the desired visual effect such as high distinctness of image (DOI). Finally, such composite coatings must also simultaneously provide a desirable balance of finished film properties such as chip performance, durability, hardness, flexibility, and resistance to environmental etch, scratching, marring, solvents, and/or acids.
Chip performance or gravel resistance is particularly important in automotive coatings, especially those intended for use on automotive components with leading edges, such as rocker panels and front bumpers. Weak or poor resistance to chipping can result in significant damage to the overall vehicle appearance and greatly reduced durability.
The prior art continues to seek an individual coating that provides improved chip resistance in a wide variety of coating systems employing various types of topcoats. More particularly, the prior art has yet to provide a way to improve chip resistance that is independent of the chemistry or molecular structure of the resin or binder component of a particular coating.
Of course, improvements in chip resistance must not be obtained at the expense of other important properties such as appearance and VOC.
In order to obtain the extremely smooth finishes that are generally required in the coatings industry, coating compositions must exhibit good flow before curing. Good flow is observed when the coating composition is fluid enough at some point after it is applied to the substrate and before it cures to a hard film to take on a smooth appearance. Some coating compositions exhibit good flow immediately upon application and others exhibit good flow only after the application of elevated temperatures.
One way to impart fluid characteristics and good flow to a coating composition is to incorporate volatile organic solvents into the composition. These solvents provide the desired fluidity and flow during the coating process, but evaporate upon exposure to elevated curing temperatures, leaving only the coating components behind.
However, the use of such solvents increases the volatile organic content (VOC) of the coating composition. Because of the adverse impact that volatile organic solvents may have on the environment, many government regulations impose limitations on the amount of volatile solvent that can be used. Increasing the percentage nonvolatile (% NV) of a coating composition or decreasing the VOC provides a competitive advantage with respect to environmental concerns, air permitting requirements, and cost.
There is thus a continuing desire to obtain thermoset coatings having an improved chip performance while still possessing the optimum balance of performance properties required by the automotive industry. This optimum balance of performance properties in the finished film must be obtained without sacrificing the Theological properties of the coating composition required for trouble-free application of the composition while still maintaining the optimum level of smoothness and appearance.
Unfortunately, the pursuit of such properties in a curable coating requires numerous experimental tests. The evaluation of chip resistance is typically conducted via a gravelometer test.
Gravelometer tests for automotive coatings generally require the placement of cured coated panels in a commercially available gravelometer. Gravel of a particular size and composition is then directed against the surface of a cold test panel. The size and frequency of resultant chipping is then evaluated, either via software or with the naked eye. Test panels for particular samples are normally done in triplicate.
However, the preparation of such gravelometer test panels continues to be a laborious and costly aspect of the development of automotive coating compositions having improved gravelometer performance.
Current automotive gravelometer testing practices typically require that all components of a cured multilayer coating system be applied and cured as required by the automotive manufacturer. These requirements are attributable to the prior art's failure to develop a method that correlates the individual gravelometer performance of a cured film of an individual component of a multilayer coating system to the gravelometer performance of the cured overall multilayer coating system.
For example, even though a set of test panels are intended solely to evaluate the gravelometer performance of particular primer compositions, each gravelometer test panel requires the application and curing of the specific primer composition followed by such topcoats as may be employed in a desired commercial multilayer coating system. The term ‘multilayer coating system’ as used herein generally refers to cured coating systems comprising a substrate, a primer, and a topcoat wherein the topcoat may comprise one or more sealers, monocoats, basecoats, color and/or effect coating compositions, clearcoat coating compositions, and combinations thereof. In one embodiment, an illustrative example of a commercially available automotive multilayer coating system will include an electrocoated steel substrate, a primer, and one or more applications of a wet-on-wet composite coating comprising a color and/or effect basecoat and a clearcoat.
As a result, the preparation of gravelometer test panels for the automotive coatings industry is normally a costly and lengthy process that requires the application, flashing, and curing of various other coating compositions in addition to the application and curing of the coating undergoing evaluation. Examples of illustrative ‘other’ coating compositions include any coatings that are utilized in multilayer automotive coating systems, i.e., sealers, basecoats, color and/or effect coating compositions, clearcoat coating compositions, and combinations thereof. The preparation of gravelometer tests panels thus continues to be a laborious and costly aspect of the development of automotive coating compositions having improved gravelometer performance.
It would therefore be advantageous to provide a method for evaluating the chip performance of a coated substrate that does not have the disadvantages of current gravelometer testing practices.
It would also be advantageous to develop a single measurement, nondestructive method for predicting the chip performance of a cured multilayer coating system by evaluating a particular property of one component of the cured multilayer coating system. It would be especially desirable if the evaluation of the one component was conducted without the use of current gravelometers or gravelometer testing practices but was predictive of the gravelometer performance of the overall cured multilayer coating system.